This invention relates to tunable lasers, and more particularly to a compression-tuned Bragg grating-based laser.
It is known in the art of fiber optics that Bragg gratings embedded in the fiber may be used in compression to act as a tunable filter or tunable fiber laser, as is described in U.S. Pat. No. 5,469,520, entitled xe2x80x9cCompression Tuned Fiber Gratingxe2x80x9d to Morey, et al and U.S. Pat. No. 5,691,999, entitled xe2x80x9cCompression Tuned Fiber Laserxe2x80x9d to Ball et al.
To avoid fiber buckling under compression, the technique described in the aforementioned U.S. Pat. Nos. 5,469,520 and 5,691,999 uses sliding ferrules around the fiber and grating and places the ferrules in a mechanical structure to guide, align and confine the ferrules and the fiber. However, it would be desirable to obtain a configuration that allows a fiber grating to be compressed without buckling and without sliding ferrules and without requiring such a mechanical structure.
Also, it is known to attach an optical fiber grating to within a glass tube to avoid buckling under compression for providing a wavelength-stable temperature compensated fiber Bragg grating as is described in U.S. Pat. No. 5,042,898, entitled xe2x80x9cIncorporated Bragg Filter Temperature Compensated Optical Waveguide Devicexe2x80x9d, to Morey et al. However, such a technique exhibits creep between the fiber and the tube over time, or at high temperatures, or over large compression ranges.
The very narrow line width ( less than 10 kHz) of single mode fiber lasers will, depending on the application, be an advantage (low phase noise) or a disadvantage (high power and narrow line width causes stimulated Brillouin scattering and hence loss). In telecom this should not be a problem since the lasers will be modulated, creating side-bands and hence effectively broadening the spectrum and increasing the threshold for Brillouin scattering.
Several fiber lasers in series or in parallel can be pumped using one semiconductor pump laser reducing the cost per fiber laser. Alternatively, parallel fiber lasers can be pumped by several pumps through a series of cross-connected couplers to form a pump redundancy scheme. With Er-lasers the pump absorption is very low and hence effectively broadening the spectrum and increasing the threshold for Brillion scattering.
Several fiber lasers in series or in parallel can be pumped using one semiconductor pump laser, reducing the cost per fiber laser. Alternatively, parallel fiber lasers can be pumped by several pumps through a series of cross-connected couplers to forma pump redundancy scheme. With Er-lasers the pump absorption is very low and hence the output power is low (xcx9c0.1 mW). This can be enhanced by a MOPA design using the residual pump power to pump an EDFA. Using Er:Yb and 980 nm pumping the pump absorption is greatly enhanced and the output power increased (xcx9c10 mW) [Kringlebotn et al., xe2x80x9cEfficient Diode-Pumped Single-Frequency Erbium: Ytterbium Fiber Laserxe2x80x9d, IEEE Photonics Techn. Lett, Vol. 5, No. 10, pp 1162-1164 (October 1993); and J. T. Kringlebotn et al., xe2x80x9cHighly-efficient, Low-noise Grating-feedback Er3+:TB3+ Codoped Fibre Laserxe2x80x9d, Eectr. Lettr., Vol. 30, No. 12, pp. 972-973, (June 1994), which are incorporated herein by reference in their entirety]. This high pump absorption can in some cases cause thermal effects resulting in mode-hopping and power saturation. Highly photosensitive Er:Yb fibers are harder to make than Er fibers.
Various tunable semiconductor lasers have been realized. DFB lasers have a limited temperature tenability (1-2 nm). Using sampled grating DBR cavities or combination of narrowband sampled grating filtering and broadband co directional filtering (using forward coupling between two parallel waveguides wide tuning ranges (xe2x88x92 greater than 40-100 nm) with relatively stable single mode operation can be realized (cf. Altitium laser). A problem with such designs is that they typically require 4 section cavities (gain, coupler, phase, reflector) with three individually/relatively controlled currents, making relatively complex and long lasers. Note that there are also various ways to make multi-wavelength/wavelength selective semiconductor laser arrays.
There are (at least) three possible FBG based single mode tunable fiber laser configurations: I) DFB, ii) DBR, and iii) sampled DBR.
DFB lasers using one phase-shifted FBG co-located with the gain medium should offer the best performance in terms of robust single mode operation, but require a highly photosensitive, high gain fiber, either Er or Er:Yb, and a relatively sophisticated FBG writing setup. DFB lasers should be able to provide the shortest grating based lasers. DBR lasers consisting of two FBG end-reflectors can be easier to realize, since separate gain fibers and grating fibers can be used (this requires low loss splicing), and the grating specs are relaxed. Mode-hopping can be a problem with DBR lasers.
Both DFB and DBR fiber lasers are continuously tunable through uniform strain of the whole cavity, including the gratings, in which case the cavity mode(s) and the Bragg wavelength are tuned equally [G. Ball and W. W. Morey, Opt. Lett., Vol. 17, pp. 420-422]. A practical tuning range in the order of 10 nm should be feasible. Both DFB and DBR fiber lasers can be designed to operate in a single polarization.
A sample grating DBR uses two sampled grating end-reflectors with comb-like reflection spectra over a wide wavelength range, and where the two gratings have different comb period. Using the Vernier effect this provides wide step-wise tuning with less compression/strain than required than for DFB/DBR lasers to get the same tuning range (a reduction by a factor of 10 probably have to be quite long (several cm) to get sufficiently strong reflection from each peak.
A fiber laser can be designed to achieve single longitudinal mode lasing, as is discussed in U.S. Pat. No. 5,305,335, entitled xe2x80x9cSingle Longitudinal Mode Pumped Optical Waveguide laser Arrangementxe2x80x9d, U.S. Pat. No. 5,317,576, entitled xe2x80x9cContinuously Tunable Single-Mode Rare-Earth Doped Pumped Laser Arrangementxe2x80x9d, and U.S. Pat. No. 5,237,576, entitled xe2x80x9cArticle Comprising an Optical Fiber Laserxe2x80x9d, which are incorporated herein by reference in their entirety.
A general fiber laser and amplifier arrangement similar to a Master Oscillator Power Amplifier (MOPA) arrangement is described in U.S. Pat. No. 5,594,747 entitled xe2x80x9cDual-Wavelength Pumped Low Noise Fiber Laserxe2x80x9d, and U.S. Pat. No. 5,666,372 entitled xe2x80x9cEmbedded Bragg Grating Laser Master-Oscillator And Power-Amplifierxe2x80x9d, which are incorporated herein by reference.
Objects of the present invention include a tunable Bragg grating-based laser that allows the grating to be compression-tuned without creep and without requiring sliding ferrules or a mechanical supporting structure for the ferrules. The laser includes at least one grating element having a large transverse dimension that advantageously provides ease of manufacturability and handling as well as provides a waveguide much less sensitive to strain and environmental changes (e.g. bending and thermal changes).
According to the present invention, a compression-tuned laser comprises a first optical waveguide. At least a portion of which has a transverse cross-section, which is continuous and comprises a substantially homogeneous material. The at least portion of the first optical waveguide has an outer transverse dimension of at least 0.3 mm. The first optical waveguide includes an inner core disposed along the longitudinal axis of the first optical waveguide, and a first grating disposed within the core along the longitudinal axis. The grating reflects a first reflection wavelength of light. A second optical waveguide includes an inner core disposed along the longitudinal axis of the second optical waveguide, and a second grating disposed within the core along the longitudinal axis. An optical fiber includes a gain material that is optically disposed between the first and second optical waveguide. At least the first optical waveguide is axially compressed to change the first reflection wavelength reflected by the first grating.
According further to the present invention, a compression-tuned laser comprises a tunable optical waveguide, wherein at least a portion of which has a transverse cross-section, which is continuous and comprises a substantially homogeneous material. The at least portion of the optical waveguide has an outer transverse dimension of at least 0.3 mm. The optical waveguide includes an inner core disposed along the longitudinal axis of the optical waveguide. The inner core includes a dopant to provide an optical gain. The optical waveguide further includes a first grating and a second grating disposed within the inner core along the longitudinal axis. The first and second gratings are spaced a distance apart, the second grating having a plurality of gratings, each of which having a respective filter function spaced spectrally. The outer transverse dimension of the optical waveguide about the first grating is different than the outer transverse dimension of the optical waveguide about the second grating.
According further to the present invention, an optical laser comprises an optical waveguide, wherein at least a portion of which has a transverse cross-section, which is continuous and comprises a substantially homogeneous material. The at least portion of the optical waveguide has an outer transverse dimension of at least 0.3 mm. The optical waveguide includes a pair of inner cores disposed along the longitudinal axis of the optical waveguide. The inner cores are spaced to minimize optical coupling therebetween. A first grating is disposed within one core along the longitudinal axis, and a second grating is disposed within the other core along the longitudinal axis. A gain element is optically disposed between the pair of inner cores.
According further to the present invention, an optical laser comprises a first optical waveguide, wherein at least a portion of which has a transverse cross-section, which is continuous and comprises a substantially homogeneous material. The at least portion of the first optical waveguide has an outer transverse dimension of at least 0.3 mm. The first optical waveguide includes an inner core disposed along the longitudinal axis of the first optical waveguide, and a first grating disposed within the core along the longitudinal axis. A reflective element reflects light at a desired wavelength band. An optical fiber includes an optical gain material, wherein the fiber is optically disposed between the first optical waveguide and reflective element. At least the first optical waveguide is axially compressed to change the first reflection wavelength reflected by the first grating.